bivith, few films have been mastered at 4K. You may find a few real 4K films currently (such as Logan or Spectre) and you'll find even more in the future. But even recent films expected to be massive hits like The Force Awakens were mastered in 2K.

4K discs of 2K films still have some benefits, namely a greater dynamic range and a greater color palette like what you'd have seen in the theater, which home theaters could not reproduce until now.